Incurable and almost always fatal, feline infectious peritonitis (FIP) is, thankfully, a relatively rare disease, affecting less than one percent of all cats presented to veterinarians for treatment. Although the condition is most prevalent by far in multicat households, animal shelters, and crowded breeding catteries, every cat owner should be aware of its viral origins, its clinical signs, and the ways in which the risk of its occurrence can be minimized.

This affliction, which primarily affects young cats (less than two years of age) and those that are 10 years of age and older, is caused by infection with the feline infectious peritonitis virus (FIPV), a mutated form of the feline coronavirus (FCoV), so named due to the microscopic appearance of its outer surface, which resembles the sun’s corona.

Coronaviruses are shed in an infected animal’s saliva and feces, and an uninfected cat can become infected by ingesting or inhaling it when coming in close contact with an infected cat or with virus-contaminated objects in the environment, such as a poorly maintained litter box. However, according to Fred W. Scott, DVM, PhD, professor emeritus of virology at Cornell University’s College of Veterinary Medicine, the most common form of transmission occurs when a persistently infected queen passes the FCoV to her offspring, usually when the kittens are five to eight weeks of age, after they have lost the temporary immunity provided to them by the queen.

Most kittens infected with FCoV do not show signs of illness following initial infection. (At most, they will experience very mild intestinal disease.) Five to 10 percent of FCoV-infected kittens may develop clinical FIP weeks, months, or even years later. Clinical FIP generally has a gradual onset over several days or weeks, with persistent fever, decreased appetite, weight loss, and an unkempt appearance.

Eventually, the disease will almost always manifest itself in either of two forms—“wet” or “dry”—which are distinguished primarily by the extent to which fluid accumulates in one or more of a cat’s body cavities. According to Dr. Scott, the wet (or “effusive”) form occurs when fluid collects within an infected cat’s body—usually in its abdomen or, in rare instances, around its heart. Fluid in the chest cavity puts pressure on the lungs, and an infected cat will have difficulty breathing. Occasionally, cats affected with effusive FIP will survive for months, but they typically will survive for only a few days to a week.

In the dry (“noneffusive”) form, there is little if any fluid accumulation. Instead, cats with this form of the disease may have clinical signs typically associated with impairment affecting an internal organ or system, such as kidney or liver failure, neurologic dysfunction, and ocular disease. Cats with noneffusive FIP may survive longer than those with the effusive form—but virtually all cats afflicted with clinical FIP will eventually die from it.

Aside from post-mortem examination or examination of biopsy tissue, says Dr. Scott, a definitive diagnosis of FIP is elusive if not impossible. A test for FCoV antibodies in an ailing cat’s blood, for example, can show only that the animal has been exposed at some point to a coronavirus, but it cannot prove beyond a doubt that the animal has FIP. Therefore, he explains, diagnosis of FIP in a living cat is most often based on tests excluding all other conditions that might be causing the clinical signs plus a series of tests that, if positive, are consistent with FIP. And treatment, at least for now, consists only of supportive care and, perhaps, efforts to alleviate an afflicted animal’s self-destructive inflammatory response to viral infection.

Although a vaccine aimed at protecting against FIP exists, Dr. Scott notes that its efficacy has proved to be of dubious value. Therefore, he advises, the most effective way in which an owner can safeguard a domestic cat—especially in a multicat household—is to prevent direct contact with an FCoV-positive cat. Meticulous hygiene, especially in the care of food dishes, litter boxes, and surrounding areas will certainly help. “You can use bleach, quartenary ammonium compounds, or any of the household disinfectants on the market,” he notes, “and you should use them daily. And try not to let your cat or cats outdoors where they might encounter an FCoV-infected cat.”

Fortunately, he notes, infections are rare among household cats. The major prevalence, by far, occurs in breeding catteries and other multicat facilities. For additional information on FIP, see Feline Infectious Peritonitis.